1. Field of the Invention
The present invention relates generally to a burst/continuous wave converting apparatus, and more particularly, to a burst/continuous wave converting apparatus for converting bursts to a continuous wave, such as a phase-locked loop (PLL) circuit, used as, for example, a color synchronizing circuit in a color television (TV) receiver or a time base corrector (TBC), a servo system of a spindle motor or a color converting circuit in a video tape recorder (VTR).
2. Description of the Prior Art
In general, a color television (TV) signal is transmitted with a carrier for a chrominance signal, i.e., a chrominance subcarrier being suppressed. Thus, when the chrominance signal is demodulated by synchronous detection in, for example, a color TV receiver, a chrominance subcarrier of 3.58 MHz must be generated using an oscillator on the side of the receiver, to be applied to a color demodulator. The frequency and the phase of the above described oscillator must be precisely controlled in response to color bursts sent superimposed on the back porch of a horizontal synchronizing signal in a video signal such that the chrominance subcarrier which is a continuous wave generated on the side of the receiver holds a correct frequency and phase for color demodulation. Therefore, in general, a PLL type color synchronizing circuit is used as a color synchronizing circuit for the color TV receiver, and a voltage controlled oscillator (VCO) is used as a chrominance subcarrier generating apparatus included therein. A continuous wave output of the VCO and the color bursts extracted from the received color TV signal are compared with each other in a phase detector, so that the frequency and the phase of the VCO are controlled in response to the comparison output. Such a PLL type color synchronizing circuit is disclosed in, for example, Japanese Patent Laying-Open Gazette No. 52285/1982.
FIG. 1 is a block diagram showing schematically a structure of a conventional color TV receiver using such a PLL type color synchronizing circuit.
In FIG. 1, a video intermediate frequency signal in a color TV signal received by a receiving antenna 1 and a tuner 2 is applied to an intermediate frequency amplifier circuit 3, to be amplified. The signal amplified in the intermediate frequency amplifier circuit 3 is further detected in a video detector circuit 4, so that a video signal is extracted. This extracted video signal is applied to a video amplifier circuit 5, to be amplified. An output of the video amplifier circuit 5 is applied to a color output circuit 6, a band-pass filter (BPF) 7, an automatic gain control (AGC) circuit 8 and a synchronizing separator circuit 9. The BPF 7 extracts a carrier chrominance signal and color bursts from the applied video signal, applies the carrier chrominance signal to a color demodulator circuit 10, and applies a composite chrominance signal comprising the carrier chrominance signal and the color bursts to a PLL type color synchronizing circuit 12. In addition, the AGC circuit 8 is responsive to a level of the video signal applied from the video amplifier circuit 5 for applying a control signal to the tuner 2 and the intermediate frequency amplifier circuit 3, to adjust the gain of the video signal. On the other hand, the synchronizing separator circuit 9 separates a horizontal synchronizing signal and a vertical synchronizing signal from the applied video signal and applies the horizontal synchronizing signal therein to a burst gate pulse generating circuit 11. The burst gate pulse generating circuit 11 responsively generates a burst gate pulse and applies the same to the PLL type color synchronizing circuit 12.
The PLL type color synchronizing circuit 12 comprises a burst gate circuit 12a, a phase detector circuit 12b, a low-pass filter (LPF) 12c, a VCO 12d (including a crystal resonator 12e), and a 1/4 frequency divider 12f. The burst gate circuit 12a receives the composite chrominance signal from the BPF 7, and extracts only the color bursts in response to the burst gate pulse applied from the burst gate pulse generating circuit 11, to apply the same to one input of the phase detector circuit 12b. The VCO 12d oscillates at an oscillating frequency 4f.sub.SC of four times a chrominance subcarrier frequency f.sub.SC. An oscillation output of the VCO 12d is divided into 1/4 by the 1/4 frequency divider 12f and then, supplied to the other input of the phase detector circuit 12b. The phase detector circuit 12b compares phases of the applied color bursts and the oscillation output of the VCO 12d, so that the comparison output is supplied to the VCO 12d through the LPF 12c as a control voltage. As a result, a chrominance subcarrier which is a continuous wave synchronized with the color bursts is extracted from the 1/4 frequency divider 12f, and applied to the color demodulator circuit 10.
The color demodulator circuit 10 extracts a color difference signal from the carrier chrominance signal using the chrominance subcarrier, and applies the same to the color output circuit 6. A color picture tube 13 is driven in response to an output signal of the color output circuit 6.
FIG. 2A is a waveform diagram showing the color bursts applied to the phase detector circuit 12b from the burst gate circuit 12a shown in FIG. 1, and FIG. 2B shows the frequency spectrum thereof.
As shown in FIG. 2A, the color bursts are a chrominance subcarrier corresponding to 8 to 9 cycles inserted into the back porch of the horizontal synchronizing signal on the side of transmission. Thus, the repetition frequency of the color bursts is equal to a horizontal frequency f.sub.H of the video signal. In addition, as shown in FIG. 2B, in the frequency spectrum thereof, sidebands appear at the intervals of horizontal frequency f.sub.H with the chrominance subcarrier frequency f.sub.SC being centered. Thus, if the capture range of the PLL type color synchronizing circuit is wide, color synchronization is achieved at a frequency other than the frequency f.sub.SC in the center, so that a correct color is not reproduced, resulting in an unclear picture.
In order to prevent such a situation, the capture range of a PLL circuit constituting the color synchronizing circuit must be made less than the repetition frequency f.sub.H of the color bursts. Thus, conventionally, the crystal resonator 12e having a large Q value has been used in the VCO. When such a crystal resonator is used, the capture range of the PLL circuit can be made approximately .+-.500 Hz, so that erroneous phase lock can be prevented. On the other hand, such a crystal resonator is expensive. In addition, the following problem occurs.
More specifically, when the above described crystal resonator having a large Q value is used as the VCO, the time difference appears between an input and an output of the VCO. Specifically, a delay of approximately 1.8 milliseconds occurs in the output. Such a delay is a period corresponding to approximately 30 horizontal lines. Color synchronization is not achieved due to such a delay, so that the hue of a picture changes, whereby the picture becomes unclear.
Due to the problems, if a resonator having a small Q value and short capture time is used as the VCO, the above described erroneous phase lock occurs, so that the PLL circuit cannot be effectively used as, for example, a color synchronizing circuit in a VTR in spite of the short capture time thereof.